1. Field of the Invention
The present invention relates to a position detection apparatus, and specifically, relates to an absolute position detection type (absolute type) photoelectric encoder.
2. Description of Related Art
There is known an absolute position detection type photoelectric encoder using an absolute (ABS) scale (JP 5553669 B). As an ABS scale pattern of the ABS scale, an M-sequence code which is one of pseudo random code sequences has been used.
With reference to FIGS. 1 to 3, a conventional absolute type photoelectric encoder is described. First, as illustrated in FIG. 1, an absolute type photoelectric encoder 100 includes an ABS scale 200 and a detection head part 300 provided to be movable relatively to the length measurement direction along the ABS scale 200.
FIG. 2 is an example of an ABS scale pattern of the ABS scale 200. As the ABS scale pattern, an M-sequence code which is one of pseudo random code sequences is used. When N number of continuous codes in the M-sequence code pattern are extracted, the pattern formed by the N number of codes appears once in a period of the M-sequence code pattern.
In the example of FIG. 2, codes of “1” and “0” are each represented by two bits in the ABS scale pattern in which the codes “1” and “0” are randomly arranged.
The two bits of the code “1” are a combination of a transmitting part (light-transmissive part) and a reflecting part. On the other hand, the two bits of the code “0” are both the transmitting parts.
Here, for the sake of description below, the transmitting part (light-transmissive part) is referred to as a dark part (or “L”), and the reflecting part is referred to as a bright part (or “H”).
The detection head part 300 includes a light source 310, a lens 320, a light receiving part 330, and a signal processing unit 400.
The light source 310 emits light toward the ABS scale 200. The light is reflected at the reflecting part of the ABS scale 200 and transmitted at the transmitting part. The reflected light enters a light receiving surface of the light receiving part 330 through the lens 320.
On the light receiving surface of the light receiving part 330, a bright and dark image pattern is formed according to the ABS scale pattern.
FIG. 3 illustrates the light receiving surface of the light receiving part 330.
A photodiode array 340 is provided on the light receiving surface of the light receiving part 330. The photodiode array 340 is formed by arranging photodiodes 341 at a pitch with which a pitch of the ABS scale pattern can be detected. Each of the photodiodes 341 constituting the photodiode array 340 includes a switch 342, and is connected to the signal processing unit 400 through the switch 342. By sequentially turning the switch 342 ON, a light receiving signal from each of the photodiodes 341 is swept.
The signal processing unit 400 includes an image acquisition unit 410 and a correlation calculation unit 420. The correlation calculation unit 420 stores design data of the ABS scale pattern as a reference pattern in advance. The correlation calculation unit 420 performs correlation calculation between a signal pattern acquired at the light receiving part 330 and the reference pattern, and obtains a position from a peak of the correlation. The absolute position on the ABS scale pattern is acquired in this manner.